The efforts of consumer device manufacturers to reduce energy consumption, weight and size of heat dissipaters, for example in the field of automobile entertainment devices, have generated a demand for power amplifiers with a greater efficiency than Class-AB amplifiers. Class-D amplifiers have been proposed to respond to these demands. Substantially, Class-D amplifiers include a DC-AC converter circuit which produces a pulse width modulated (PWM) output signal. This pulse width modulated output signal drives power switches, which in turn switch a load which may implement a passive lowpass filter for reconstructing the amplified audio signal, of which the actual load (e.g. a speaker) may be a part. The analysis of the behavior of a single ended amplifier with an analog input and a PWM output (a Class-D amplifier) is described in the paper "Analysis of a quality Class-D amplifier", F. A. Himmelstoss, et al., I.E.E.E. Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996.
On the other hand, the increasing interest in digital audio signal processing, will make it more convenient to use digital amplifiers rather than analog amplifiers. Therefore, many manufacturers of audio equipment require Class-D power amplifiers capable of handling analog audio signals and digital audio signals. According to a common technique, to process an analog input signal requires the generation of a reference waveform, commonly a triangular or a saw-tooth signal, whose generation is relatively critical and at any rate requires a certain circuit complexity.
An example of a Class-D amplifier capable of selectively handling analog audio signals and digital audio signals, is described in U.S. Pat. No. 5,672,998. The amplifier uses a circuit for generating a PWM signal from an analog audio signal, separated from the feedback loop of the power amplifier itself. Even in this case, in order to generate a PWM signal, a triangular reference signal, produced by a dedicated generator, is employed.